Traffic signals are usually controlled by a local traffic signal controller. The signal controller is programmed to provide control signals for the red, yellow and green lights of the signal. The control signals are applied to switching circuits. In response to the control signals, the switching circuits provide current, usually 120 volts AC, to the particular traffic signal light that is to be illuminated. Typically, a distinct switching circuit is associated with each traffic signal light.
The signal controller continuously monitors the output signals of each switching circuit to determine whether a switching circuit has failed. Such failure could produce a dangerous traffic condition. For example, a malfunctioning switching circuit could cause green lights to be displayed to intersecting traffic lanes.
Whenever a conventional signal controller detects a switching circuit failure, the controller departs from the programmed normal operation, and the traffic signal is controlled to flash red lights in all directions. This "flash mode" is maintained until the traffic signal is serviced.
Placing a traffic signal in a flash mode upon detection of a switching circuit malfunction is useful for the immediate protection of motorists traveling through that intersection. At many intersections, however, the flash mode will greatly interfere with traffic flow, making it necessary to service the signal as soon as possible after the malfunction is detected.